There have been widely known internal combustion engines that can switchably use two or more kinds of fuels according to the situation, and widely known internal combustion engines that can change the blend ratio between fuels. As a control apparatus for such an internal combustion engine, there has been a known apparatus that detects an inappropriate combustion state with a sensor, and changes the blend ratio or the ignition timing of fuels so that the inappropriate combustion state changes to an appropriate combustion state (Patent Literature 1). Also, as an internal combustion engine that switchably uses two or more kinds of fuels, there has been a known engine that switches the fuel being used to a fuel with a smaller amount of air pollutants when determining that the air pollutants in the exhaust is likely to become higher in density (Patent Literature 2).